I'm looking for a way for customise the excerpt length automatically, to something more brief, say, ten words.

I'm aware of the custom field, but adding an excerpt to that extra field (for each post) would be very time-consuming since I post at least once a day - I would have to count out the correct length of excerpt, and then paste it in, every time I post.

Same issues as Simon here... I'm running 2.8.4, and none of the above solutions seem to have any effect in my functions file... Probably I'm missing something obvious (a brain) but nonetheless, help appreciated...

I also tried the same solution that is shown in the codex:
add_filter('excerpt_length', 'my_excerpt_length');
function my_excerpt_length($length) {
return 250; // Or whatever you want the length to be.
}

And it does not appear to work or make any difference on WP 2.8.4 ... the excerpt remains at 55!

i've been chasing down this bug . in my case its the eventcalendar3 plugin that is removing the default filter wp_trim_excerpt and overriding it with its own filter ( which is hardwired to 55 words ) . can i suggest that you try disabling that plugin if you have it or search all your plugins for ones that add_filters to excerpt hooks ?

here is my fix ( to the event calendar plugin ) . i'll point the developer to this post too .